The latest update seems to prevent any workarounds, other than editing the Bookmarks.plist directly.
This is somewhat dangerous, although this will make a backup you can restore. You need to open terminal and make sure it has Full Disk Access under System Settings > Privacy and Security > Full Disk Access
Close Safari and run each command
# backup bookmarks
cp -p ~/Library/Safari/Bookmarks.plist ~/Desktop/Bookmarks.plist.backup
# convert to xml
plutil -convert xml1 ~/Library/Safari/Bookmarks.plist -o ~/Desktop/Bookmarks.xml
# remove all dateAdded
sed -i '' '/<key>dateAdded<\/key>/{N;d;}' ~/Desktop/Bookmarks.xml
# convert back to binary
plutil -convert binary1 ~/Desktop/Bookmarks.xml -o ~/Library/Safari/Bookmarks.plist
Then restart Safari and all recently added should be gone. If there is an issue then
# restore backup
cp -p ~/Desktop/Bookmarks.plist.backup ~/Library/Safari/Bookmarks.plist